Studies involving B lymphocytes and their immunoglobulin receptors will be extended to T cells where the character of the T cell receptor for antigen remains obscure. Tetanus toxoid will be used in immunized individuals and attempts will be made to isolate from labelled solubilized T cell membranes the material that combines with toxoid antigen. Suppressor factors against T cell reactions will also be studied. Emphasis will be placed on material derived from certain lymphoid lines.